1. Technical Field to Which the Invention Belongs
The present invention relates to the stabilization of a lysine solution, and in particular to the stabilization of a lysine solution by incorporating a predetermined amount of an acid radical therein.
Incidentally, the acid radical is the residue of an acid molecule from which one or more hydrogen atoms capable of being replaced by metal(s) have been removed, and it constitutes the negative moiety of a salt. The acid radical often refers to an atomic group such as the acid radical (SO.sub.4) of sulfuric acid, but a single atom such as Cl in hydrogen chloride is also referred to as an acid radical, this nomenclature being followed in this specification.
Furthermore, the solubility of lysine refers to the equilibrium maximum amount of lysine soluble in unit water amount of a lysine solution.
Still furthermore, the product of the molar number of an acid radical multiplied by its valence refers, e.g., to the molar number of the acid radical when the acid radical consists of a monovalent acid radical only, or to the product of the molar number of the acid radical multiplied by 2 as its valence when the acid radical consists of a divalent acid radical only, or to the sum of the molar number of a monovalent acid radical and the product of the molar number of a divalent acid radical multiplied by 2 as its valence when the acid radical consists of both monovalent and divalent acid radicals.
2. Prior Art
As is well-known, lysine is used as one of the essential amino acids in a large amount as a feed additive for producing a feed for domestic animals such as cattle, pigs and chickens as well as an industrially produced mixed feed. In so doing, however, lysine is not used as crystals in the free and pure form but used usually in the form of monohydrochloride, because lysine is readily soluble in water, is hardly crystallized in the free form, exhibits strong moisture absorption (i.e., is strongly hygroscopic), absorbs carbon dioxide from the air, possesses a significantly unpleasant odor and is liable to degradation. In the present specification, lysine means lysine in the L-form (i.e. L-lysine) except quotations from the prior art literature and is referred to as "lysine base" when it is intended to emphasize that lysine is not in a salt form but in the free form (free lysine)
The monohydrochloride is a compound which is stable, not liable to degradation and readily crystallized, and does not possesses properties such as moisture absorbing and unpleasantly smelling. However, its production involves the problems of additional raw material costs and process costs. Further, the use of lysine monohydrochloride for enriching lysine in feeds and industrially produced mixed feeds increases not only the content of lysine but also the content of chlorides in the mixed feeds, but this is generally undesired. Further, the ratio of lysine to chlorides is also important. This is because, in feeding experiments with crystalline amino acid feeds, it is often observed that the positive effects of the amino acid added are completely lost, when the amino acid is used as the hydrochloride, whereby the chloride excess, in turn, occurs. For these reasons, lysine in the chloride-free form has been strongly desired for the purpose of nutritional enrichment for feeds and industrially produced mixed feeds.
Under such technical background, the JP-B 3-5783 patent document discloses an invention titled "A process for producing a feed and an industrially produced mixed feed enriched with lysine wherein an aqueous L-lysine solution containing 30 to 80 weight % L-lysine is used".
With respect to the findings on which this prior art invention is based, said document states, "It has been found that when L-lysine is used in the form of an aqueous solution for enrichment, L-lysine, which is unstable in the free form, can be used for enrichment of feeds and mixed feeds. It has been unexpected that the aqueous solution is stable even at high temperatures in storage for a long time. A comparative experiment has showed that an aqueous 70% L-lysine solution is not colored at all after storage at 50.degree. C. for 6 weeks. An experiment on by-products which may be formed depending on the condition has also indicated minus. As compared with the product in solution, crystalline L-lysine is evidently colored during this storage. Formation of by-products has been confirmed in an analytical experiment. Accordingly, the found behavior described above has been absolutely unexpected, because general experience teaches that products liable to degradation are more easily degraded in a dissolved form than in a crystalline form". The patent document makes comments on the advantages of the invention, "An aqueous solution of lysine can be produced easily as compared with crystalline L-lysine monohydrochloride, and the aqueous solution has the advantage that the content of chlorides in mixed feeds is not increased and the solution can be accurately metered even in a small amount. When a mixed feed is to be enriched with L-lysine, the L-lysine as a liquid composition does not usually require inevitable production of a pre-mixture having L-lysine at a correspondingly high concentration. The aqueous solution of lysine can, directly at a desired concentration, be mixed uniformly with other ingredients previously present at desired concentrations in a mixed feed, for example by sprinkling the lysine solution in a mixing cooker." Concerning the unexpectedness of the invention, the document additionally states, "Although L-lysine is referred to as one kind of amino acid which is destroyed first of all by heat treatment not only in materials but also in feeds and mixed feeds--this free L-lysine is added in the form of an aqueous L-lysine solution, and is unexpectedly stable even in feeds and mixed feeds. Degradation and/or reaction does not occur due to other feed ingredients. In a feeding experiment, an aqueous L-lysine solution and L-lysine monohydrochloride exhibit the same action insofar as these are used in the same molar amount".
The patent document describes, concerning the concentration of the aqueous lysine solution of the invention disclosed therein, "To achieve the desired improvement as to weight increase and utilization of feed, various amounts of L-lysine should be added to each mixed feed in order to compensate for the content of natural L-lysine in feed protein. To use an aqueous L-lysine solution according to the present invention, the amount thereof is generally 0.01 to 5%, relative to the weight of the finished mixed feed, depending on the concentration of the L-lysine in the solution. In this case, a solution with a L-lysine content of 30 to 80% by weight, advantageously 50 to 70% by weight, is particularly preferable.", and in respect of the process for producing the same, the document states, "Such a solution is obtained by dissolving L-lysine in a corresponding amount of water. In commercially producing L-lysine, it is naturally easy and suitable to produce an aqueous solution containing L-lysine at a desired concentration during the commercial production of L-lysine. This evidently facilitates production and does not so cost as in production of particularly L-lysine monohydrochloride."
For nutritionally enriching feeds or industrially produced mixed feeds with lysine, lysine in the form of a liquid composition possesses such various advantages as described in JP-B 3-5783 supra, but the liquid composition of lysine described in the patent document involves the problem that the lysine is easily precipitated as free form lysine crystals as the temperature of the atmosphere drops during storage. Precipitation of such crystals will, in turn, cause clogging of pipes for transferring the liquid composition of lysine in factories or during shipping and unloading or make it difficult to transfer the liquid composition of lysine at a predetermined concentration, thus worsening the handling of the liquid composition of lysine.
To sum up, when lysine is used in the form of a liquid composition, a lysine base solution has been used in many cases because of higher solubility than in a lysine salt solution such as lysine hydrochloride solution, lysine sulfate solution etc. However, from the conventional lysine base solution with a concentration of not less than 50% by weight, as described in JP-B 3-5783 supra, lysine base is crystalized when the temperature drops during storage, to cause, e.g., clogging of piping, thus worsening the handling in some cases.